Borrowing the terminology of computational learning theory [1]},
a representation class (or language) for a concept class \(\mathcal {F}\)  is a set of strings \(\mathcal {R}\) 
defined over some (possibly infinite) alphabet of symbols. \(\mathcal {R}\)  is associated with two surjective functions, namely,
a mapping \(\llbracket \cdot  \rrbracket  : \mathcal {R} \rightarrow \mathcal {F}\) , called representation scheme,
and a mapping \(|\cdot |: \mathcal {R} \rightarrow \mathbb {N}\) , capturing the size of each representation.
Any string \(\rho \in \mathcal {R}\)  for which \(\llbracket \rho  \rrbracket  = f\)  is called a representation
of the concept \(f\) .
